MKV file has too many frames
Your source file has inconsistent frame rates in the container and the video stream (see this explanation of tbc, tbn, and tbr), so that may throw Blender off. Try running your video file through ffmpeg without re-encoding the streams, and set the frame rate explicitly:

ffmpeg -i yourfile.mkv -c:v copy -c:a copy -r 30 newfile.mkv